In accordance with the prior art methods, blow molding articles are produced by extruding a hollow plastic substance while in a semi-molten state (hereinafter referred to as the "parison"), clamping the parison within a mold, blowing air into the inside of the parison to form a blow molded article, then externally cooling the article by passing cold water through the molds. In addition, it is known to increase the cooling effect by using the latent heat and the sensible heat of liquefied gas at low temperatures such as liquid carbon dioxide and liquid nitrogen (LIN) by directing the liquefied gas into the interior of the blow molding article during the cooling stage.
When liquid carbon dioxide is used as a coolant, great difficulty is experienced in decreasing the pressure within the feed conduit to the mold so that it is below the triple point of carbon dioxide, e.g., 4.2 kg./cm.2 gauge. Furthermore, if liquid carbon dioxide is injected directly into the parison at the commencement of the blowing cycle, it precipitates in the form of snow or dry-ice and attaches itself to the inner wall of the blow molded article while it is still at high temperatures. This results in great strain on the wall of the article as a result of the non-uniformity of the thermal stress. Therefore, the usual practice is first to introduce air into the blow molded article above the triple point pressure of carbon dioxide and then to introduce the liquefied gas such as carbon dioxide or LIN into the pre-formed article. In this prior art method, in which two fluid mediums are used for this coolant step, namely compressed air and liquefied carbon dioxide, complications exist because the feed conduit and the feed nozzle become plugged from the freezing of the moisture in the compressed air. The prior art also teaches the use of liquid carbon dioxide for blowing and cooling. For the reasons described above, additional equipment is required in this prior art method so that the liquid carbon dioxide is vaporized at the beginning of the blowing step before it can be used as the blowing gas.
It is also known in the prior art to use for cooling blow molded articles LIN which has a more intensive cooling action over liquid carbon dioxide. According to the disclosure of such prior art methods, LIN is passed from a suitable storage through a LIN supply valve, a downstream pipe and a blow mandrel where it is vaporized before it is passed into the parison during the beginning of the blowing step. The LIN in this case is passed through a single supply line. The disadvantage of this prior art process is that the possibility exists of forming ice on the outer surface of the LIN feed conduit which results in a great loss in its ability to transmit heat. As a result, the LIN does not become vaporized and flows into the parison at fluctuating temperatures which produces strain on the resulting blow molded article or causes surface defects from cold shock when the blow molding operation is continuously cycled. Specifically it has been found in the case of blowing large containers, it is necessary to vaporize LIN with a heater or to use a vaporizer to assure that the nitrogen is supplied in its gaseous state. In addition, the prior art methods have been plagued with the problem of the inability to quantitatively feed the vaporized LIN for the blowing step. Therefore, it has been found that when blow molding large containers in which LIN is used, air is first used as the blowing gas and subsequently LIN is introduced to cool the preformed article. This method has also the same disadvantages as those described above in connection with the use of liquefied carbon dioxide. The feed conduit between the blow pin used to introduce the blowing gas and the LIN supply valve is comparatively long in order to introduce heat storage capacity. As a result, the loss of LIN by the end of the blowing step is considerable resulting in another disadvantage of these prior art processes.